Cylindrical air filters are used in conjunction with a wide variety of air breathing machinery, including trucks and cars, gas turbines, bulldozers and other heavy equipment. Typically, this type of filter employs a perforated metal frame and an enclosed, generally cylindrical pleated paper filter medium having inside and outside surfaces. Some of these filters feature "inside" filtering wherein the dirt and debris are collected on the inside of the cylindrical filter. Others feature "outside" filtering wherein particulates are collected on the outside cylindrical surface. Most automobiles employ relatively inexpensive air filters that are simply discarded and replaced when they become dirty. However, trucks and other types of heavy equipment usually employ air filters that are relatively large and expensive to replace. Accordingly, various techniques have been developed to clean and reuse these large filters. Most often, they are washed in a suitable solvent or cleaning solution. Such washing tends to weaken the paper filter medium so that it is significantly less effective when the filter is replaced.
In an attempt to overcome the disadvantages of cleaning solvents, a number of known devices have employed a "dry" cleaning system wherein dirt and debris are dislodged from the filter and then collected by a vacuum or other suction means. For example, in the device disclosed by U.S. Pat. No. 4,808,234, air pressure is applied simultaneously to the inside and outside cylindrical surfaces of the filter to dislodge dirt from the outside surface. Suction collects this dislodged dirt. As the air pressure is applied, the filter is rotated horizontally about its longitudinal axis. This apparatus is restricted to cleaning outside filters. There is no way that dirt from an inside filter can be collected. Additionally, it is relatively difficult to mount the filter in the machine of Patent No. '234. The operator must manually reach into the cabinet of the apparatus and hold the filter while securing it to a spindle. This is awkward and hazardous and may violate various governmental safety codes and regulations.
In the filter cleaning apparatus disclosed by U.S. Pat. No. 5,143,529, the air filter is mounted upright in a cabinet. High pressure, opposing air nozzles are mounted for vertical movement along the inner and outer surfaces of the filter to dislodge debris therefrom. Similarly, inner and outer vacuum nozzles are mounted adjacent to and movable with the air nozzles for drawing in the dislodged debris. Although this apparatus facilitates handling and mounting of the filter and cleans both inside and outside filters, it also exhibits a number of serious disadvantages. For one thing, it cannot properly clean many narrow, tapered filters that are now commercially available because the inside nozzle often cannot move freely through the central opening of such filters. Additionally, the upright filter rotates during the cleaning process. As a result, when an inside filter is cleaned, centrifugal force causes the dirt and debris to be pushed tightly into the interior pleats. This makes cleaning of the inside surface more difficult. Cleaning is also complicated because as the interior debris falls from the paper filter, much of it drops into corners in the floor of the filter, which are inaccessible to the vacuum. Therefore, the filter is not effectively and completely cleaned.
In neither of the above described devices can the user select either inside or outside cleaning alone. In Patent No. '234 only outside cleaning is possible. In Patent No. '529 both types of cleaning are perfumed on every filter, although virtually all air filters normally require only one type of cleaning (either inside or outside). Moreover, the known systems are not easily adapted for different sizes of filters. Typically, they are programmed to operate for a predetermined time that cannot be automatically shortened or lengthened, as required, for smaller or larger filters.